Auditory Processing Disorder (APD) (previously known as "Central Auditory Processing Disorder" (CAPD)) is not a hearing impairment (i.e., a person with APD usually has nothing wrong with his or her ears), but an inability to process what is heard. APD is an umbrella term that describe a variety of problems with the brain that can interfere with processing auditory information.

Both of these documents provide the first comprehensive definitions of APD in the respective countries, and a platform for future research and development of diagnostic systems and support programs. (There are links to both documents included in the External Links section below.) They cover the various causes of Auditory Processing Disorder including both the genetic causes and the acquired causes (such as severe ear infections and severe head injuries).

Aspects of auditory processing which may be affected by APD include "auditory discrimination", the ability to distinguish between similar sounds or words; "auditory figure-ground", the ability to distinguish relevant speech from background noise; and "auditory memory", the ability to recall what was heard.

The first research into APD began in 1954 with Helmer Myklebust’s study, "Auditory Disorders in Children".[1] Myklebust’s work suggested auditory processing disorder was separate from language learning difficulties. His work sparked interest in auditory deficits after acquired brain lesions affecting the temporal lobes[2][3] and led to additional work looking at the physiological basis of auditory processing,[4] but it was not until the late seventies and early eighties that research began on APD in depth.
In 1977, a conference about APD started a new series of studies focussing on APD in children.[5][6][7][8][9] Virtually all tests currently used to diagnose APD originate from this work. These early researchers also invented many of the auditory training approaches, including interhemispheric transfer training and interaural intensity difference training. This period gave us a rough understanding of the causes and possible treatment options for APD.
Much of the work in the late nineties and 2000s has been looking to refining testing, developing more sophisticated treatment options, and looking for genetic risk factors for APD. Scientists have worked on improving behavioral tests of auditory function, neuroimaging, electroacoustic, and electrophysiologic testing.[10][11] Working with new technology has led to a number of software programs for auditory training.[12][13] With global awareness of mental disorders and increasing understanding of neuroscience, auditory processing is more in the public and academic consciousness than ever before.[14][15][16][17][18]

APD is a difficult disorder to detect and diagnose. The subjective symptoms that lead to an evaluation for APD include an intermittent inability to process verbal information, leading the person to guess to fill in the processing gaps. There may also be disproportionate problems with decoding speech in noisy environments.

APD has been defined anatomically in terms of the integrity of the auditory areas of the nervous system.[19] However, children with symptoms of APD typically have no evidence of neurological disease and the diagnosis is made on the basis of performance on behavioral auditory tests. Auditory processing is "what we do with what we hear",[20] and in APD there is a mismatch between peripheral hearing ability (which is typically normal) and ability to interpret or discriminate sounds. Thus in those with no signs of neurological impairment, APD is diagnosed on the basis of auditory tests. There is, however, no consensus as to which tests should be used for diagnosis, as evidenced by the succession of task force reports that have appeared in recent years. The first of these occurred in 1996.[21] This was followed by a conference organized by the American Academy of Audiology.[22] Experts attempting to define diagnostic criteria have to grapple with the problem that a child may do poorly on an auditory test for reasons other than poor auditory perception: for instance, failure could be due to inattention, difficulty in coping with task demands, or limited language ability. In an attempt to rule out at least some of these factors, the American Academy of Audiology conference explicitly advocated that for APD to be diagnosed, the child must have a modality-specific problem, i.e. affecting auditory but not visual processing. However, an ASHA committee subsequently rejected modality-specificity as a defining characteristic of auditory processing disorders.[23]

The issue of modality-specificity has led to considerable debate among experts in this field. Cacace and McFarland have argued that APD should be defined as a modality-specific perceptual dysfunction that is not due to peripheral hearing loss.[24][25] They criticise more inclusive conceptualizations of APD as lacking diagnostic specificity.[26] A requirement for modality-specificity could potentially avoid including children whose poor auditory performance is due to general factors such as poor attention or memory.[24][25] Others, however, have argued that a modality-specific approach is too narrow, and that it would miss children who had genuine perceptual problems affecting both visual and auditory processing. It is also impractical, as audiologists do not have access to standardized tests that are visual analogs of auditory tests. The debate over this issue remains unresolved. It is clear, however, that a modality-specific approach will diagnose fewer children with APD than a modality-general one, and that the latter approach runs a risk of including children who fail auditory tests for reasons other than poor auditory processing.

Another controversy concerns the fact that most traditional tests of APD use verbal materials.[27] The British Society of Audiology[28] has embraced Moore's (2006) recommendation that tests for APD should assess processing of non-speech sounds.[27] The concern is that if verbal materials are used to test for APD, then children may fail because of limited language ability. An analogy may be drawn with trying to listen to sounds in a foreign language. It is much harder to distinguish between sounds or to remember a sequence of words in a language you do not know well: the problem is not an auditory one, but rather due to lack of expertise in the language.

In recent years there have been additional criticisms of some popular tests for diagnosis of APD. Tests that use tape-recorded American English have been shown to over-identify APD in speakers of other forms of English.[29] Performance on a battery of non-verbal auditory tests devised by the Medical Research Council's Institute of Hearing Research was found to be heavily influenced by non-sensory task demands, and indices of APD had low reliability when this was controlled for.[30]

Depending on how it is defined, APD may share common symptoms with ADD/ADHD, Specific language impairment, Asperger syndrome and other forms of autism. A review showed substantial evidence for atypical processing of auditory information in autistic children.[31] Dawes and Bishop noted how specialists in audiology and speech-language pathology often adopted different approaches to child assessment, and they concluded their review as follows: "We regard it as crucial that these different professional groups work together in carrying out assessment, treatment and management of children and undertaking cross-disciplinary research."[32] In practice, this seems rare.

Acquired APD is not a unitary disorder. Any damage to or dysfunction of the central auditory nervous system can cause auditory processing problems.[33][34] For an overview of neurological aspects of APD, see Griffiths.[35]

In the majority of cases of developmental APD, the cause is unknown. An exception is acquired epileptic aphasia or Landau-Kleffner syndrome, where a child's development regresses, with language comprehension severely affected.[36] The child is often thought to be deaf, but normal peripheral hearing is found. In other cases, suspected or known causes of APD in children include delay in myelin maturation,[37] ectopic (misplaced) cells in the auditory cortical areas,[38] or genetic predisposition.[39] In a family with autosomal dominantepilepsy, seizures which affected the left temporal lobe seemed to cause problems with auditory processing.[40] In another extended family with a high rate of APD, genetic analysis showed a haplotype in chromosome 12 that fully co-segregated with language impairment.[41]

Hearing begins in utero, but the central auditory system continues to develop for at least the first decade.[42] There is considerable interest in the idea that disruption to hearing during a sensitive period may have long-term consequences for auditory development.[43] One study showed thalamocortical connectivity in vitro was associated with a time sensitive developmental window and required a specific cell adhesion molecule (lcam5) for proper brain plasticity to occur.[44] This points to connectivity between the thalamus and cortex shortly after being able to hear (in vitro) as at least one critical period for auditory processing. Another study showed that rats reared in a single tone environment during critical periods of development had permanently impaired auditory processing.[45] ‘Bad’ auditory experiences, such as temporary deafness by cochlear removal in rats leads to neuron shrinkage.[42] In a study looking at attention in APD patients, children with one ear blocked developed a strong right-ear advantage but were not able to modulate that advantage during directed-attention tasks.[46]

In the 1980s and 1990s, there was considerable interest in the role of chronic Otitis media (middle ear disease or 'glue ear') in causing APD and related language and literacy problems. Otitis media with effusion is a very common childhood disease that causes a fluctuating conductive hearing loss, and there was concern this may disrupt auditory development if it occurred during a sensitive period.[47] Consistent with this, in a sample of young children with chronic ear infections recruited from a hospital otolargyngology department, increased rates of auditory difficulties were found later in childhood.[48] However, this kind of study will suffer from sampling bias because children with otitis media will be more likely to be referred to hospital departments if they are experiencing developmental difficulties. Compared with hospital studies, epidemiological studies, which assess a whole population for otitis media and then evaluate outcomes, have found much weaker evidence for long-term impacts of otitis media on language outcomes.[49]

The National Institute on Deafness and Other Communication Disorders[50] state that children with Auditory Processing Disorder often:

have trouble paying attention to and remembering information presented orally, and may cope better with visually acquired information

have problems carrying out multi-step directions given orally; need to hear only one direction at a time

have poor listening skills

need more time to process information

have low academic performance

have behavior problems

have language difficulties (e.g., they confuse syllable sequences and have problems developing vocabulary and understanding language)

have difficulty with reading, comprehension, spelling, and vocabulary

APD can manifest as problems determining the direction of sounds, difficulty perceiving differences between speech sounds and the sequencing of these sounds into meaningful words, confusing similar sounds such as "hat" with "bat", "there" with "where", etc. Fewer words may be perceived than were actually said, as there can be problems detecting the gaps between words, creating the sense that someone is speaking unfamiliar or nonsense words. Those suffering from APD may have problems relating what has been said with its meaning, despite obvious recognition that a word has been said, as well as repetition of the word. Background noise, such as the sound of a radio, television or a noisy bar can make it difficult to impossible to understand speech, since spoken words may sound distorted either into irrelevant words or words that don't exist, depending on the severity of the auditory processing disorder.[51] Using a telephone can be problematic for someone with auditory processing disorder, in comparison with someone with normal auditory processing, due to low quality audio, poor signal, intermittent sounds and the chopping of words.[23] Many who have auditory processing disorder subconsciously develop visual coping strategies, such as lip reading, reading body language, and eye contact, to compensate for their auditory deficit, and these coping strategies are not available when using a telephone.

As noted above, the status of APD as a distinct disorder has been queried, especially by speech-language pathologists[52] and psychologists,[53] who note the overlap between clinical profiles of children diagnosed with APD and those with other forms of specific learning disability. Many audiologists, however, would dispute that APD is just an alternative label for dyslexia, SLI, or ADHD, noting that although it often co-occurs with these conditions, it can be found in isolation.[54]

SLI is diagnosed when a child has difficulties with understanding or producing spoken language for no obvious cause. The problems cannot be explained in terms of peripheral hearing loss. The child is typically late in starting to talk, and may have problems in producing speech sounds clearly, and in producing or understanding complex sentences. Some theoretical accounts of SLI regard it as the result of auditory processing problems.[55][56] However, this view of SLI is not universally accepted, and others regard the main difficulties in SLI as stemming from problems with higher-level aspects of language processing. Where a child has both auditory and language problems, it can be hard to sort out cause-and-effect.[56]

Similarly with developmental dyslexia, there has been considerable interest in the idea that for some children reading problems are downstream consequences of difficulties in rapid auditory processing. Again, cause and effect can be hard to unravel. This is one reason why experts such as Moore[27] have recommended using non-verbal auditory tests to diagnose APD.

It has also been suggested that APD may be related to cluttering,[57] a fluency disorder marked by word and phrase repetitions.

If, as is commonly done, APD is assessed using tests that involve identifying, repeating or discriminating speech, then a child may do poorly because of primary language problems.[32] In a study comparing children with a diagnosis of dyslexia and those with a diagnosis of APD, they found the two groups could not be distinguished.[56][58][59] obtained similar findings in studies comparing children diagnosed with SLI or APD.[60][61] The two groups had very similar profiles. This raises the worrying possibility that the diagnosis that a child receives may be largely a function of the specialist they see: the same child who would be diagnosed with APD by an audiologist may be diagnosed with SLI by a speech-language therapist or with dyslexia by a psychologist.[27]

There is a lack of well-conducted evaluations of intervention using randomized controlled trial methodology. Most evidence for effectiveness adopts weaker standards of evidence, such as showing that performance improves after training. This does not control for possible influences of practice, maturation, or placebo effects. Recent research has shown that practice with basic auditory processing tasks (i.e. auditory training) may improve performance on auditory processing measures[62][63] and phonemic awareness measures.[64] Changes after auditory training have also been recorded at the physiological level.[65][66] Many of these tasks are incorporated into computer-based auditory training programs such as Earobics and Fast ForWord, an adaptive software available at home and in clinics worldwide, but overall, evidence for effectiveness of these computerised interventions in improving language and literacy is not impressive.[67] One small-scale uncontrolled study reported successful outcomes for children with APD using auditory training software.[68]

Treating additional issues related to APD can result in success. For example, treatment for phonological disorders (difficulty in speech) can result in success in terms of both the phonological disorder as well as APD. In one study, speech therapy improved auditory evoked potentials (a measure of brain activity in the auditory portions of the brain).[69]

While there is evidence that language training is effective for improving APD, there is no current research supporting the following APD treatments:

ASHA APD Technical Report Jan 2005 (An Acrobat download of 20 pages of A4) This report is the result of some 5 years discussion to find a unified definition of APD for the USA, which was started by the first Bruton Conference in 2000.

UK Medical Research Council's APD pamphlet Oct 2004 The UK MRC published this pamphlet as a working document to define APD for the UK to herald their first 5-year APD research program, funded by the UK government. The links provides either a download of the pamphlet, or it can be read online.

Auditory Processing Disorder in the UK (APDUK) web site includes online and downloadable copies of the UK Medical Research Council's APD pamphlet and much more information about APD (with input from Adult APDs) and related invisible disabilities.